1. Field of the Invention
The present invention relates generally to mechanical screens adapted to separate from a flow of water debris or waste conveyed thereby.
It is aimed more particularly at mechanical screens which use at least one filter chamber including a chassis which is in the general form of a frame and a filter panel which is disposed transversely in the opening of the chassis, to which it is appropriately attached, whether the filter panel is in one piece or divided into separate filter panels appropriately disposed side-by-side.
A mechanical screen of this kind can form the filter apron of a chain filter, for example.
2. Description of the Prior Art
It then includes a plurality of filter chambers articulated to each other in an endless loop, in an arrangement like that described in U.S. Pat. No. 4,889,629.
It can instead be a rotary drum, for example, or even a static filter.
When, in the case of a chain filter or a rotary drum, a mechanical screen of this kind is mobile, each of its filter chambers operates like a bucket, temporarily retaining and entraining with it debris or waste held back by its filter panel until, on passing a cleaning device directing a contraflow of water through the filter panel, the debris or waste is separated from the filter panel and eliminated.
Be this as it may, one problem to be overcome in the manufacture of mechanical screens including filter chambers of this kind relates to the need to fasten the filter panel appropriately to the chassis.
At present the filter panel is usually a two-dimensional metal mesh or a two-dimensional perforated plate. In either case it has the disadvantage of being sensitive to corrosion, and attaching it to the chassis invariably entails the use of metal parts which are also sensitive to corrosion.
The attachment is usually performed by means of nuts and bolts, using metal bolts to clamp the screen between one frame fastened to the chassis and another frame formed of metal bars, for example.
The same applies in practice when, as described in the aforementioned U.S. Pat. No. 4,889,629, the filter panel forms a three-dimensional structure which, although it is usually made of metal, nevertheless has the advantage that it can be made from a synthetic material if required.
At present the chassis used for such filter panels are usually made of metal, for example annealed steel or stainless steel.
These chassis are inevitably costly and they are never totally resistant to corrosion.
To overcome this drawback, it has already been proposed to make the chassis from a material that is inert to corrosion, for example a synthetic or composite material.
Even so, the filter panel is attached to a chassis of this kind in the conventional way, using nuts and bolts.
The cost of the nuts and bolts, which in practice must be made of stainless steel, the cost of the bars forming the other frame associated with the nuts and bolts, which must also be made of stainless steel, and the cost of the labor for assembling the structure constitute an important part of the total cost of a mechanical screen.
Furthermore, given the various metal components employed, which at present include the filter panel itself, the structure remains highly vulnerable to corrosion, especially in stagnant water.
A general object of the present invention is an arrangement which avoids these drawbacks and has other advantages.